Conventionally, a coolant circuit for cooling a heating unit mounted on a vehicle is provided with a reserve tank for absorbing a change in volume of coolant caused by a change in temperature thereof (see, for example, JP-A-2005-120906). In recent years, a predetermined clearance is required to be provided between an engine component and a bonnet so as to absorb impact on the bonnet for the protection of pedestrians. This imposes a stricter restriction on installing of components in an engine room, and thus requires reduction in size of the reserve tank.
Increase in size of the engine, in number of control components, or the like tends to reduce a space for installing of the components in the engine room. Thus, a contour of the reserve tank is complicated so as to enable ensuring a necessary capacity effectively using an empty space. Further, a pipe connected to the reserve tank is run in a narrow space, which imposes the stricter restriction on positions of an inflow portion and an outflow portion of the reserve tank.
However, some small-sized reserve tanks or vertically thin reserve tanks cannot sufficiently ensure the depth of coolant retained. Moreover, some reserve tanks are formed to have a complicated contour that interrupts the flow of the coolant flowing into the tank. In such a reserve tank, a liquid level may be disturbed due to a flow of the coolant, thereby causing air to be trapped in the coolant. The circulation of the coolant with the air trapped therein through the coolant circuit may reduce cooling performance of a heat exchanger provided in the coolant circuit, or a lifetime of a water pump to generate abnormal noise in a water pump.